It’s a great idea, heating your garage with a Reznor unit heater it shall do a lot of good things for your garage and items stored in it. By keeping the heat above 38° it will keep the garage foundation from heaving in the cold winter months. Also if the garage is attached to the house, the house benefits by heating the garage the common wall is now warm for the house interns saving you heat on said house wall.
Having it warmer also helps your vehicles start faster as well as better engine oil viscosity, because the engine block is starting warm.
Using a compact ceiling gas heater works great. We have installed several of these units. You can bolt them to the roof framing members and or floor joist system. You can mount them up tight or drop them down to a lower height if you have a 9’- 14’ high garage ceiling. By installing the unit at the back of the garage and blow air cross the garage to the far front corner this will get the greatest way to stabilize the temperature in the garage. This will also dry your car off and the floor. By keeping your garage temperature at or above 38° you should never have the garage floor flaking or popping of concrete from the winter weather.
This furnace also can be vented both out the wall and/or through the roof of the garage. Giving is a way to really maximize your opportunity for the best installation.